1. Field of the Invention
The present invention relates to a method of transferring wafers into a boat and a heat treatment apparatus provided with a system for transferring the wafers into the boat. More particularly, it relates to a method and an apparatus for transferring semiconductor wafers into the boat in a boat loading/unloading section of the vertical type CVD apparatus.
2. Description of the Related Art
Semiconductor devices have been more and more highly integrated these days and the process of manufacturing the semiconductor devices is therefore asked to meet more requisites. Various heat treatment steps are included in the semiconductor devices manufacturing process and semiconductor wafers are heated and cooled at these heat processing steps.
Conventionally, those apparatuses in which the CVD film forming, diffusing and oxidizing processes were carried out were mainly of the horizontal type. In the case of the horizontal CVD apparatus, however, air is likely to be included in the process atmosphere at the boat loading time. Naturally-oxidized film is thus formed on each of the wafers to thereby reduce the productivity of the wafers.
The vertical CVD apparatus has been therefore used more and more to carry out the CVD film forming process. In the case of the vertical CVD apparatus, the wafer boat is loaded into and unloaded from the process tube through the bottom opening thereof and this makes it difficult to include air into the process atmosphere.
The vertical CVD apparatus of this type has a boat loading chamber under the process tube. The boat loading chamber (or lower chamber) is provided with an elevator system for moving the wafer boat up and down.
When the wafers are to be transferred from a cassette to the boat, it is needed that the adhering of particles to the wafers is prevented. It is therefore preferable that the wafers transferring operation is carried out in the lower chamber if possible.
When the wafers transferring system is to be housed in the lower chamber, however, the volume of the lower chamber must accordingly be made large. This takes a long time in purging the lower chamber with inactive gas. If the lower chamber is not fully purged with inactive gas, the wafers of high temperature unloaded into the lower chamber will be oxidized and naturally-oxidized film unnecessary for carrying out the film forming process will be formed on each of the wafers. Therefore, it is needed that the lower chamber is made as small as possible.
Further, when the wafers transferring system is housed in the lower chamber, it can be seriously deteriorated by heat radiated from the process tube, the boat and the wafers. It may be protected from this radiated heat by a cover member, but when covered by the cover member, it becomes large in size. It is therefore needed that the lower chamber is made large in size. This causes the loading and unloading of the wafers and the boat to take a long time.